Introduction to mold

Molds, a subset of fungi, are everywhere on our planet. Fungi are found in every ecological niche, and are necessary for the recycling of organic building blocks that allow plants and animals to live. Included in the group fungi are yeasts, molds and mildews, as well as large mushrooms, puffballs and bracket fungi that grow on dead trees. Fungi need external organic food sources and water to be able to grow. Molds can grow on cloth, carpets, leather, wood, sheet rock, insulation and human foods when moist conditions exist (Gravesen et al., 1999).

Because molds grow in moist or wet indoor environments, it is possible for people to become exposed to molds and their products, either by direct contact on surfaces, or through the air, if mold spores, fragments, or mold products are aerosolized. Many molds reproduce by making spores, which, if they land on a moist food source, can germinate and begin producing a branching network of cells called hyphae. Molds have varying requirements for moisture, food, temperature and other environmental conditions for growth. Indoor spaces that are wet, and have organic materials that mold can use as a food source, can and do support mold growth. Mold spores or fragments that become airborne can expose people indoors through inhalation or skin contact.

Mold spores are fungal reproductive cells about the same size as pollen grains. They can occur in various colors and shapes, such as round, spheroid, banana-shaped or tadpole-shaped. They can occur in enormous quantities, and at all times of the year. Mold spores can be found and generated at serious levels indoors, as well as out.

Fungi can invade healthy individuals and can cause a variety of effects. The most common response is allergies (runny nose, sneezing, sinus congestion, and skin rashes). Allergies result from inhaling mold spores. When environmental conditions become conducive, many molds develop fungal hyphae, small appendages containing spores. These spores are analogous to plant seeds and are spread in billions when air currents pass over the hyphae. Even dead fungi are capable of causing allergic symptoms.

Mold spores can be airborne, and get indoors through doors, windows or cracks and crevices, or be carried in from the outdoors on shoes and clothing. Building materials that were left outside before use can harbor living mold spores for many years. Indoor environments are never entirely free of molds. As a general rule of thumb, in a "healthy" building the concentration of spores and the mix of mold species tend to be similar to outdoor environment levels.

If buildings are air-conditioned, or windows and doors are kept closed in summer, the concentration of spores within should even be lower than outside levels. High moisture (above 70.0% relative humidity) in a building will invariably lead to mold, mildew, or other microbial growth.

Microbrial growth requires four things:

• A nutrient source (found in most building materials)
• Proper temperature (usually found indoors)
• Mold spores (ubiquitous in ambient air)
• Water

Some molds also produce toxins which are thought to be useful in killing competing molds in their vicinity. These toxins can also have deadly effects on humans when ingested, inhaled or in contact with the skin. The fungi that produce toxins are known as toxigenic fungi. Many fungi produce secondary toxic metabolites which can produce adverse health effects (mycotoxicoses) in animals and human. Metabolites are collectively known as mycotoxins. A 1990 World Health Organization publication on mycotoxins indicated that there are more than 200 mycotoxins produced by a variety of common fungi. Historically, mycotoxins are a problem to farmers and food industries and in Eastern European and third world countries. However, many toxigenic fungi, such as Stachybotry chartarum (also known as Stachybotrys atra) and species of Aspergillus and Penicillium, have been found to infest buildings with known indoor air and building-related problems. Many indoor air quality problems have been traced to the growth of fungus in buildings. Almost without exception, these buildings have usually had chronic water or moisture problems.

Molds can have an impact on human health, depending on the nature of the species involved, the metabolic products being produced by these species, the amount and duration of individual's exposure to mold parts or products, and the specific susceptibility of those exposed. Health effects generally fall into four categories: Allergy, Infection, Irritation and Toxicity.